The invention relates to a device for the measurement of the moisture of harvested crop, the device including a measurement capacitor equipped with two electrodes between which crop can be positioned and connected to an inductor to define a resonance circuit, and a process for the measurement of the moisture of a harvested crop.
An arrangement that operates on a capacitive measurement principle for the measurement of moisture of harvested crop is shown in U.S. Pat. No. 4,584,522. A container is filled with harvested crop of unknown moisture, and the moisture of the crop can be determined on the basis of the change in the capacitance of an electrode in the form of a capacitor plate with respect to a second electrode. A dry crop has a relatively low relative permittivity and produces only a small change in capacitance compared to an empty container, whereas a wet crop with high relative permittivity produces a larger change in capacitance. For this measurement the capacitor with unknown capacitance C is charged with a predetermined voltage and is discharged by cutting off the source of the charge over a resistance R, where the voltage applied after a certain time applied to RC module is a measure of the capacitance C.
DE 1300316 B describes an arrangement for the continuous measurement of bulk material in which the dielectric losses of the measurement capacitor are used as a measure of the water content of the material to be measured. Hence a parasitic (Ohmic) resistance of the capacitor charged with high frequency alternating current is measured.
Furthermore resonance measurement circuits have been proposed for measurement devices for the determination of moisture. In the measurement arrangements of this class described by W. Lxc3xcck in the book xe2x80x9cFeuchtigkeit, Grundlagen, Messem, Regelnxe2x80x9d (Moisture, Fundamentals, Measurement, Control) page 221 and following (Publisher R. Oldenbourg, Munich 1964), the capacitance of a measurement capacitor is determined by the measurement of the voltage on an LC parallel oscillating circuit at a frequency at which the slope of the leading or trailing edges of the oscillating circuit response is maximized. Alternatively the measurement capacitor is inserted into the oscillator as a frequency determining element.
The known moisture measurement arrangements have the disadvantage that the measurements depend upon the absolute value of the voltage applied to the measurement capacitor. The measurement voltage is applied to the measurement capacitor in the form of a direct current voltage or an alternating current voltage of constant frequency. The indicated value of the measured moisture therefore depends directly upon the measurement voltage. However it is problematic whether the measurement voltage applied to the capacitor can be held constant in order to obtain reproducible, reliable results. Therefore the measurement arrangements are relatively imprecise.
The problem to be overcome by the invention is seen as the improvement of the precision of an arrangement for the measurement of the moisture of harvested crop.
A feature of the invention includes varying the frequency of the alternating current measurement signal which is applied to the resonance circuit and detecting a parameter of the resonance circuit at various frequencies. Since the frequency dependent parameter of the resonance circuit depends among other things upon the capacitance of the measurement capacitor, the moisture content of the crop to be measured can be determined on the basis of the measured frequency dependent parameter.
By varying the frequency and detecting the frequency dependent parameter, the absolute value of the measured signal no longer represents a source of error, since only the frequency dependency, not the absolute value of the signal, is relevant for the further evaluation. Therefore the measurement device according to the invention operates very precisely.
The parameter of the resonance circuit that is detected most effectively is its impedance. It would also be conceivable to detect the phase shift between a measurement signal that is applied to the oscillating circuit over a high resistance value, and the voltage present at the oscillating circuit as parameter of the resonance circuit. The phase shift is also dependent upon the capacitance of the measurement capacitor and permits a determination of the moisture of the crop to be measured.
For the determination of the impedance of the resonance circuit, a signal is applied to a voltage divider circuit, one arm of which includes the resonance circuit. In one embodiment, the signal is applied to the resonance circuit via resistor or capacitor, and the amplitude of the signal is measured at the resonance circuit. The voltage detected at the resonance circuit depends upon the impedance of the resonance circuit.
The measurement of the impedance of the resonance circuit can be performed in particular in such a way that it is connected through a constant current source with the measurement signal. The constant current source may be, in particular, a constant alternating voltage source that is connected by means of a sufficiently large resistor to the resonance circuit, so that the current through the resonance circuit is constant and is independent of its impedance; then the voltage at the resonance circuit is a direct representation of its impedance. Alternatively a current could also be measured that flows through the resonance circuit. This could employ a shunt resistor on which a voltage is detected that is proportional to the current and is also a representation or measure for the impedance of the resonance circuit. In this case the measurement signal is applied to the resonance circuit through a constant voltage source.
The resonance circuit is preferably a parallel oscillating circuit. In comparison to a series oscillating circuit, which fundamentally could also be utilized, the advantage lies in the fact that the resonant frequency is independent of parasitic resistances connected in parallel that can be caused by surface moisture of the measured crop.
On the basis of the frequency-dependence of the parameter detected, the resonant frequency of the resonance circuit is preferably determined which, for a known and fixed inductance L, is a measure of the capacitance C of the measurement capacitor. The capacitance, in turn, permits a determination of the dielectric constant of the harvested crop contained in the measurement capacitor, which is a function of the moisture of the harvested crop. Hence the moisture can also be determined without difficulty. The resonant frequency is located at that frequency at which the impedance of the resonance circuit is a maximum (parallel oscillating circuit) or a minimum (series oscillating circuit); however other criteria for the determination of the resonant frequency are conceivable, such as the phase shift. In the evaluation of the moisture of the crop by software it is optional whether in the calculation of the moisture intermediate calculation steps are used in which the resonant frequency and/or the capacitance of the capacitor and/or the dielectric constant xcex5 are determined explicitly, or whether the moisture is determined directly on the basis of the measured parameters or on the basis of a frequency, capacitance, dielectric constant or any appropriate value derived in any way from the measured parameters.
Since the temperature at which the measurement is performed as well as the type of the harvested crop have an effect upon the dependence between the dieletric constant and the moisture, the moisture is preferably determined on the basis of a table in which for the ambient temperature, which is detected by means of an appropriate sensor, and/or the actual type of crop (particularly type of grain crop) that can be provided as input by the user, a moisture value is assigned to the measured resonance frequency. Instead of a moisture value that is a function of the resonance frequency, the table can also store the moisture as a function of the capacitance C, the dielectric constant, or any desired other value derived from these values. Such tables can be calibrated by measurements with known parameters.
Furthermore the frequency of the measurement signal which is applied to the resonance circuit can be varied continuously or in steps. A continuous scanning of the frequency range (wobbling) can be generated by analog or digital circuit techniques by means of a triangular or saw tooth-shaped control signal that is applied to a voltage controlled oscillator (VCO). The step-wise scanning of the frequency range can be produced by a step-shaped control voltage of the VCO, which can also be attained by analog or digital switching techniques. A purely digital generation of the measurement signal by means of a digital-analog converter is also conceivable. A preferably pre-determined frequency range is scanned which includes the expected resonant frequency of the resonance circuit. It is also conceivable that several scans be performed of which a first one uses relatively large steps and a second covers only a smaller region in order to cover the resonance retrieved in the first scan, but uses considerably smaller steps, so that the resonant frequency is determined precisely.
Besides the determination of the resonant frequency, the measurement process according to the invention also permits the determination of the width of the resonance curve of the resonance circuit. The wider the response curve, the greater the damping by parasitic resistances that are connected in parallel to the resonance circuit. Parasitic resistances of this kind are usually caused by surface moisture of the crop to be measured that produces an Ohmic resistance between the electrodes of the measurement capacitor. An obvious solution therefore is to evaluate the width of the resonance curve for the sake of the determination of the surface moisture. It does not matter which width of the resonance curve is determined (base width, half value width, etc.). The measured surface moisture can be displayed by means of an indicator arrangement. When a threshold value is exceeded, an error signal can be transmitted, since a value above threshold indicates the crop is obviously too moist to be harvested.
The invention can preferably be applied in agricultural machines, in particular in combines, also known as combine harvesters, or forage harvesters, in which the moisture of the harvested crop (normally grain crops) can be determined. The measured moisture values, as well as surface moisture values, if applicable, can be used for yield mapping and be stored geographically referenced or transmitted by remote data transmission to a central office.